1. Field
The invention relates a turbine system for a steam turbine power plant and in particular a coal fired steam turbine power plant operating together with a post combustion carbon capture plant. The invention in particular relates to an arrangement for the optimisation of turbine train configuration to facilitate integration with a post combustion carbon capture system.
2. Description of the Related Art
A typical turbine train for power generation in coal fired plants comprises a high pressure turbine, an intermediate pressure turbine, one or more low pressure turbines and an electrical generator. A typical general configuration of the turbine train for power generation in coal fired plants comprises a high pressure turbine, an intermediate pressure turbine, one or more low pressure turbines and an electrical generator (with excitation system) with rigid couplings connecting the rotors of individual modules which rest on bearing supports in the “tandem compound” arrangement, which is illustrated in FIG. 1. This is familiar to practitioners as prior art.
Other arrangements of turbine train for power generation are generally known.
For example in the case of power generation in nuclear plants and/or steam generation plants with large requirements of generation capacity, an alternate turbine train is conceptualised as a “cross compound” arrangement where steam is drawn from a source upstream of the low pressure turbine(s) and distributed to multiple low pressure turbines not all of which are connected to the same rotor train. This ensures rotor-dynamic stability, reduces loss in efficiency due to excessive thermal expansion between sealing fins on account of last stage blade length limitations in available families of low pressure turbines to handle the required flows, and reduces the multiplicity of such turbines mounted on the same shaft. The low pressure turbine on the different shaft is rigidly connected to an electrical generator on the different shaft, but which is a part of the same rotor train. This is illustrated in FIG. 2 and is also familiar to practitioners as prior art.
Power generation from gas turbines in combined cycle mode of operation often involves a (single shaft) turbine train comprising of a gas turbine (with compressor), electrical generator and steam turbine modules connected with flexible couplings, as typically illustrated in FIG. 3 and is also familiar to practitioners as prior art.
The above arrangement is vital for the operation of the gas turbine in “open cycle” mode, without the steam turbine being necessarily put into operation as in the “combined cycle” mode. This arrangement allows for the economic usage of a single electric generator for a wide range of loads; besides providing wide-ranging flexibilities in operations, testing, erection and commissioning.
The steam turbine modules depicted in FIG. 3 may be replaced by another gas turbine drive for use as a pure “peaking” plant. This provides all the advantages mentioned above in terms of economy and flexibility.
Most of the energy used in the world today is derived from the combustion of fossil fuels, such as coal, oil, and natural gas. Post-combustion carbon capture (PCC) is a means of mitigating the effects of fossil fuel combustion emissions by capturing CO2 from large sources of emission such as thermal power plants which use fossil fuel combustion as the power source. The CO2 is not vented to atmosphere but is removed from flue gases by a suitable absorber and stored away from the atmosphere.
It is known that CO2 can be separated from a gas phase, for example being the flue gas of a thermal power plant, by means of absorption by suitable absorption medium, for example absorbent in liquid phase, typically in aqueous solution. Gas is passed through the absorption medium under conditions of pressure and temperature optimised for removal of substantially all the carbon dioxide. The purified gas is then directed for further processing as necessary. The absorption medium rich in CO2 is subjected to a stripping process to remove the CO2 and regenerate the absorption medium.
Typically this process involves regenerative heating of the medium. The CO2 rich medium is maintained at high temperature, which may be at or near boiling point of an absorbent liquid phase under pressure. The heat necessary is typically obtained when the system is used in association with a thermal power plant by supplying steam from the LP turbine system. At higher temperatures the medium will release the absorbed CO2. Regenerated medium may be drawn off for reuse. The released CO2 may then be collected for example for sequestration. The condensate product of the steam used to supply regenerative heat is returned to the steam generation system.
In case of a tandem compound steam turbine plant such as illustrated in FIG. 1, with integrated post combustion carbon capture, it is necessary to extract steam for the regeneration of the lean solvent from a point upstream of the low pressure steam turbines, for example, in the case where the system comprises HP, IP and LP turbines or turbine sets with combined HP/IP modules, from the vicinity of the IP/LP crossover.
The diverting of steam away from the LP turbine for carbon capture results in a deviation from the design practice embodied in low pressure steam turbines in conventional power stations where this requirement to divert steam for carbon capture is not present.
The pressure upstream of low pressure steam turbines is dictated by a characteristic area available for flow inside the turbine (the “swallowing capacity”) and the prevailing condenser vacuum which is dictated by prevalent ambient conditions. Any reduction in flow through low pressure steam turbines results in a reduction of upstream pressure. This reduction in upstream pressure adversely effects the loading of last stages of blades in the intermediate pressure turbine which are typically located upstream of the low pressure steam turbine in the same rotor train, connected with rigid couplings.
The resulting differential between the downstream pressure of the intermediate pressure steam turbine and the upstream pressure of the low pressure steam turbine as a consequence of flow extraction for carbon capture is implemented in the form of a flow restricting device which might for example be a valve in the cross-over line between the intermediate pressure and low pressure steam turbines or a diaphragm valve located within the low pressure steam turbine upstream of the flow passage for the blades.
The consequence of steam extraction from the cross-over line results in a deviation from the optimum design of the low pressure steam turbines for conventional power stations without carbon capture. Both operating efficiency and flexibility are adversely affected, as explained in FIG. 4. Earlier efforts to partially counteract the same and to provide an improved stability of operating regime for a tandem compound steam turbine integrated with a post combustion carbon capture plant have been elaborated in United Kingdom Patent Application No. 1010760.5, entitled “Operation of Steam Turbine and Steam Generator Apparatus with PCC”.
Notwithstanding such modifications the integration of a tandem compound steam turbine system with a post combustion carbon capture plant has detrimental ramifications for steam turbine driven plants in terms of capital costs and operating expenditure.
For a steam turbine driven power plant with integrated carbon capture, there is a projected reduction of about 20% from the available power generation capacity due to steam extraction upstream of the low pressure steam turbines. This not only results in oversizing of the low pressure steam turbines and the electrical generator (resulting in increased capital costs), but also results in considerable deterioration of operating efficiency and flexibility of low pressure steam turbines (the extent of such deterioration being lower for the electrical generator).
The oversizing of the low pressure turbines and generator may require the consequent oversizing of a significant number of other capital intensive equipment like turbine foundation, low pressure feed water heaters, condenser, circulating water system, cooling towers, electrical protection system, isolated phase bus ducts, etc. This not only results in significant capital expenditure on oversized components which are expected to perform well below their design duties; but in operating them considerably away from their optimum design level, significantly increase the actual operating expenditure of the power plant.
It is evident from the above that there exist certain specific requirements for integrating a post combustion carbon capture plant with a steam turbine train which cannot be optimally satisfied with a tandem compound configuration. The consequences of non-optimal design lead to greater degrees of deviation in case of carbon capture involving multiple units of steam turbine driven power plants.
For example European Patent Publication EP2333255 describes a fossil fuel combustion thermal power system with carbon capture plant. FIG. 7 illustrates such a typical post combustion carbon capture plant. However the reference concerns itself with part-load operation and protection of the main thermal power system with respect to malfunctioning or trip scenarios of the carbon capture plant. It does not address more fundamental issues like design efficiency with respect to the uncoupling of the flows of the low pressure turbine specifically for flows diverted to the carbon capture plant.
Problems include:
                1. Ensuring optimal selection of low pressure steam turbines for both regimes of operation (with or without carbon capture) simultaneously achieving best efficiency with wide operational flexibility.        2. Avoiding oversizing of low pressure steam turbines for multiple regimes of operation (with or without carbon capture) with resultant increase of capital costs and with predicted underperformance.        3. Optimising the selection of low pressure turbines for various percentages of carbon capture in relation to specified/target capacities.        4. Avoiding oversizing of capital intensive equipment typically described above together with the electrical generator and associated systems in view of major reductions in load dispatch (which could exceed 20%).        